1. Field of the Invention
This invention relates to a fuel control system for integral rocket-ramjets which is automatically actuated upon transition from rocket to ramjet propulsion and which provides the limiting functions necessary to prevent the engine, air inlet and vehicle from operating in unacceptable regions. The fuel control modulates thrust by controlling fuel flow to perform the functions of ramjet light-off, inlet margin limiting, maximum fuel-to-air ratio limiting, vehicle velocity or Mach number limiting, and lean blowout limiting.
2. Description of the Prior Art
The concept of ramjet propulsion for vehicles was initiated in the early 1900's, and practical development evolved in the 1940's. Since then numerous advances have been made in this technology, and applications to advanced missiles is predicted for the future.
Rockets have been known for many centuries and led to practical applications during World War II and thereafter, with both solid fueled and liquid fueled rockets used for space exploration.
The marriage of the ramjet and rocket took place during the 1960's when the integral rocket-ramjet (IRR) was developed for missile applications. In 1967 the low altitude short-range missile LASRM was developed. The basic IRR is a combined propulsion system. Ramjet fuel is sealed off from the rocket fuel so that the IRR starts out as a pure rocket engine using rocket fuel in the ramjet combustion chamber, and a rocket nozzle inside the ramjet nozzle. During the rocket boost, the ramjet air inlets are typically covered with blow-off fairings and the air openings to the combustion chamber are sealed off with blow-off plugs. When the rocket fuel burns out, the blow-off fairings, inlet plugs and rocket nozzle are ejected, leaving a ramjet propulsion system which is then ignited. Numerous configurations of IRR's have been developed, and this invention is applicable to any IRR in which a liquid fuel is fed to the ramjet combustion chamber and ignited upon termination of the rocket phase.
The basic ramjet consists of a supersonic air inlet, a combustor, a fuel supply system and an exhaust nozzle. The supersonic air inlet admits air to the engine, reduces the air velocity, and interfaces with the combustor which develops combustor pressure. The combustor adds heat and mass to the air by burning the fuel, and thereby increases combustor pressure. The nozzle converts combustion chamber pressure tod kinetic energy to produce thrust.
The fuel to the ramjet is supplied from a storage tank by pumping or pressurization. A fuel control modulates fuel flow to prevent the engine, inlet and vehicle from operating in unacceptable regions. The fuel control must permit thrust modulation over as large a range as is practical without exceeding the operating constraints to optimize vehicle performance. The fuel control matches fuel flow with airflow to maintain the fuel-to-air ratio within limiting values for both lean and rich mixtures. Operation is closely inter-related with conditions in both inlet and combustor. The fuel control also must maintain an appropriate initial flow of fuel during transition from rocket to ramjet operation, control inlet pressure margin, and limit flight Mach number. A ramjet fuel control may be considered an air inlet control in that it positions the shock wave at a desirable location in the ramjet inlet and meters fuel as required to maintain that shock position for the inlet limiting portion of the flight envelope.
Numerous fuel controls fo ramjets are known in the prior art, most of which have disadvantages such as inability to provide proper fuel flow over the large range of operating conditions encountered during high performance ramjet operation. More specifically, prior art ramjet fuel controls did not take into account the effect of the shock wave produced at the air inlet and thus often encountered operating conditions where vehicle performance deteriorated.
The present invention improves high performance ramjet operation by scheduling fuel via a novel fuel control system which prevents the engine, inlet and vehicle from operating in inefficient or unacceptable regions, and provides thrust modulation over a wide range of operating conditions. The control is adaptable to a wide variety of vehicle configurations, and has the advantages of low cost, high reliability and low weight and volume. The basic control parameters are derived using electronic devices, and are adapted to digital implementation, while the actual fuel metering is performed by proven hydromechanical controls.
It is therefore an object of this invention to provide a ramjet fuel control which meters fuel to the ramjet combustor to ensure safe ram burner light-off and transition from rocket to ramjet propulsion.
Another object of this invention is a ramjet fuel control which meters fuel to prevent the air inlet from operating in the unstable subcritical region via an open loop inlet margin control.
A further object of this invention is a ramjet fuel control which provides both maximum and minimum fuel-to-air ratio limits for optimizing operation and preventing lean burner blowout.
Another object of this invention is a ramjet fuel control which provides a closed loop maximum vehicle velocity or Mach number control by metering fuel flow.